A magnetic device is a device that uses magnetic material arranged in a defined structure for shaping and directing magnetic fields in a predetermined manner to achieve a desired electrical performance. The magnetic fields in turn act as the medium for storing, transferring and releasing electromagnetic energy.
Magnetic devices most typically consist of a core composed of a magnetic material having a magnetic permeability greater than that of the surrounding medium (typically air). The core is of a volume and may have legs of a desired cross-sectional area. The core (or each leg thereof) is surrounded and excited by a plurality of windings of a desired number of turns and carrying an electrical current. Because of the high permeability of the magnetic core, magnetic flux produced by the windings is confined almost entirely to the core; the flux follows the path the core defines and the flux density is essentially consistent over the uniform cross-sectional area of the core.
Many magnetic devices contain air gaps in their core legs to reduce their tendency to saturate. In such devices, the core is divided into core-halves that mate at corresponding core faces. When the length of the air gap between the core-halves is less than the cross-sectional area of the adjacent core faces, the magnetic flux is essentially constrained to reside in the core and the air gap and is continuous throughout the magnetic device. The resulting reluctance of the magnetic device is an aggregate function of the length of the air gap, the cross-sectional area of the core legs, the number of windings surrounding each of the core legs and the permeability of the magnetic material constituting the core.
To ensure that a particular core configuration is as generic as possible to the widest range of possible applications, prior art cores were provided with legs of uniform cross-sectional area and shape. The designers of such generic cores reasoned that the magnetic performance of a single, generic core could be adapted to a particular application by varying the number of windings around, and the length of the air gaps for, each core leg.
In practice, however, high volume production of magnetic devices having varying air gap lengths for each leg has proven tedious and troublesome, requiring manual labor and resulting in high manufacturing costs and unacceptable rejection rates. On a traditional high volume production line, a premolded winding assembly containing predetermined numbers of windings for each leg and provided with uniform leg apertures therethrough is registered on the uniform legs of a generic core half. A nonmagnetic material, such as paper, is manually glued onto the exposed leg faces of the core half to establish the various air gaps. Since the gap of each leg is of a different length, however, each core face is covered with a paper of different thickness. Finally, a second core half is glued in place over the first core half and air gap paper, completing the core and the magnetic device as a whole.
Unfortunately, if the wrong thickness of paper is used for even one core leg, the magnetic performance of the device is altered, often rendering it useless for the intended purpose. Further, the winding assembly with its uniform leg apertures may be inadvertently reversed with respect to the core halves. For example, in a three-leg core, the windings for leg 1 may be misplaced on leg 3 and vice versa. In devices requiring windings that vary by leg, inadvertent winding reversal with respect to the core also alters device performance.
Such deviations in magnetic device performance may substantially degrade the operation of, for instance, push-push DC/DC converters employing an isolation transformer. Such converters have, as a desired objective, low output ripple current. To achieve low ripple current, discrete inductors are used at the output to provide the necessary filtering. The problem with employing such discrete inductors is that the inductor devices are bulky and expensive.
An alternative to providing a discrete inductor involves the provision of offset tapped secondary windings in the isolation transformer. U.S. Pat. No. 5,327,333 to Boylan et al., issued Jul. 5, 1994, and entitled "Push Push DC-DC Reduced/Zero Voltage Switching Converter with Off-Set Tapped Secondary Windings," discloses such a circuit. However, to achieve zero output ripple current, a discrete inductor is still necessary for filtering the ripple at input voltages other than the input voltage for which the circuit is specifically designed. Therefore, this circuit is disadvantageous in that it employs the modified isolation transformer and a discrete inductor and thereby further increases the cost and size of the converter.
Other circuits combine the output filter inductor and the isolation transformer into one integrated magnetic device. The integrated magnetic device behaves as a combined transformer-inductor, thereby providing both voltage transformation and ripple filtering. U.S. Pat. No. 5,353,212 to Loftus, issued Oct. 4, 1994, and entitled "Zero-Voltage Switching Power Converter with Ripple Current Cancellation," discloses the advantages of such a circuit. However, while integrated magnetic devices provide a viable solution for push-push DC/DC converter circuit designs, the integrated magnetic device must be compact, cost effective and mass producible to allow its use in quantity production of push-push DC/DC converter circuits. Such devices employing varying air gaps and reversible winding assemblies are subject to the manufacturing difficulties described above.
Accordingly, what is needed in the art is a fundamental improvement in the design of cores for magnetic devices to eliminate the problems of inadvertent variations in air gap length and reversal of the winding assembly with respect to the core to allow such magnetic devices to be produced reliably on a large scale.